1. Field of the Invention
The present invention relates in general to techniques for non-invasively inspecting ferromagnetic parts, such as cylindrical or tubular members, and in particular relates to techniques for inspecting ferromagnetic cylindrical and tubular components through utilization of magnetic particles to identify stray flux which corresponds to defects in the ferromagnetic component.
2. Description of the Prior Art
Magnetic particle inspection techniques are one type of non-invasive inspection technique which may be utilized to locate surface and some sub-surface defects in ferromagnetic components. In magnetic particle inspection techniques, the ferromagnetic component is exposed to a magnetic field, which causes magnetic field flux lines to extend through the ferromagnetic component. Defects or aberrations in the ferromagnetic component will generate strong leakage fields, particularly if the defect is approximately perpendicular to the lines of the magnetic field flux. Dry or wet ferromagnetic particles are applied to the ferromagnetic component, are attracted by the leakage field, and collect at the site of the defect. This accumulation of magnetic particles indicates the location, size, and shape of the defect or aberration in the ferromagnetic component.
A broad overview of magnetic particle inspection techniques may be found in the following technical references, which are incorporated by reference herein as if fully set forth:
(1) "Nondestructive Testing Handbook", Second Edition, edited by T. J. Schmidt, K. Skeie, and P. Mclntire, published by the American Society for Nondestructive Testing, Volume 6, Part 9, pages 405-420, entitled "Oil Field Applications of Magnetic Particle Testing"; PA1 (2) "NDE Handbook" (ISBN 0-408-04392-X) edited by Knud G. Bering, Chapter 17, entitled "Magnetic Particle Examination", published in an English-language edition by Butterworths in 1989. PA1 (3) "Nondestructive Testing", authored by Warren J. McGonnagle, Chapter 10, entitled "Magnetic Methods", published by McGraw-Hill Book Company in 1961. PA1 (4) "A Survey of Electromagnetic Methods of Nondestructive Testing", authored by W. Lord, and published by Plenum Press in 1980 in a book entitled "Mechanics of Nondestructive Testing" (ISBN 0-306-40567-9), edited by W. W. Stinchcomb, J. C. Duke, Jr., E. G. Hennecke II, and K. L. Reifsnider. PA1 (1) the end caps impede the operator's view of the full exterior surface of the tubular member while the field is active; PA1 (2) the end caps also obstruct the operator's view of the interior surface of the tubular member while the current is being applied to the tubular member; PA1 (3) with this approach, the tubular member cannot be rotated while the field is active, making it difficult for the inspector to quickly inspect all exterior surfaces of the tubular member; PA1 (4) passing current directly through the tubular member can cause damage to the tubular member through undesirable electrical current arcs; PA1 (5) it is difficult to pass a great number of tubular members through a workstation in assembly line fashion, since the end caps must be coupled to each tubular member; and PA1 (6) in this approach, for safety reasons, the operator cannot inspect the tubular member while the magnetic fields are active, since the electrical current passing through the tubular member can be dangerous to the operator. PA1 (1) the operator's view of the most intense interaction between the induced stray magnetic field and the magnetic particles is obscured, since the most intense magnetic field is provided in only the lowermost portion of the tubular member while it is adjacent the bin of magnetic particles; therefore, detection of defects is entirely reliant upon the attraction of magnetic particles to the ferromagnetic tubular member and maintenance of the magnetic particles in that position while the tubular member rotates to allow a view of the region which was previously obscured; PA1 (2) accordingly, the operator will never see the most direct and intense interaction between the stray magnetic field and the magnetic particles, introducing a substantial opportunity for failure in detection of all defects on the outer periphery of the tubular member; PA1 (3) the fact that the tubular member must be passed adjacent the bin of magnetic particles slows the inspection process; PA1 (4) the utilization of dry magnetic particles necessitates that cleaning operations be performed to remove the particles to prevent the particles from interfering with the operation of the tubular, and in particular to prevent the interference of the magnetic particles with make up of gas-tight threaded engagements with other tubular members; PA1 (5) this technique does not allow for the inspection of any interior surface of the tubular member, such as the periphery of any central bore which extends through the tubular member; and PA1 (6) this technique requires the utilization of rather large U-shaped electromagnets, which are expensive.
Two particular techniques merit detailed consideration. One technique is found in U.S. Pat. No. 4,931,731 which issued to Jenks on Jul. 5, 1991, and which is entitled "Magnetic Particle Inspection Apparatus With Enhanced Uniformity of Magnetization". The other technique is found in U.S. Pat. No. 4,694,247, which issued to Meili et al. on Sep. 15, 1987, and which is entitled "Method and Apparatus Including a Cushion of Pulverulent Magnetic Material for Stray Field Magnetic Testing of Ferromagnetic Parts".
In the technique of U.S. Pat. No. 4,931,731, two magnetic fields are generated: a magnetic field which passes longitudinally over the exterior surface of a tubular member, and a magnetic field which is concentrically disposed about the tubular member along its entire length. The longitudinal magnetic field is generated by passing a current through windings which are wound about a cylindrical core. The concentric magnetic field is generated by utilizing a current source to pass current directly through the tubular member. End pieces are coupled to each end of the tubular member to connect the tubular member within an electrical circuit with the current source. The current source produces a pulse train of current, which passes through the material of the tubular member and causes the concentric magnetic field to be generated about the tubular member.
There are several serious disadvantages with this approach, including:
The inspection technique of U.S. Pat. No. 4,694,247 is in general directed to a method and apparatus for detecting defects in the outer surface of ferromagnetic parts through utilization of pulverulent magnetic material and stray magnetic fields induced in the ferromagnetic part through application of a longitudinal magnetic field and a traverse magnetic field. In this technique, U-shaped electromagnets are utilized to generate a traverse magnetization of the part and in particular a lower portion of the part. A second U-shaped magnet is utilized to generate a longitudinal magnetic field which particularly affects a lower portion of the part. A bed of dry magnetic particles are provided in a bin which engages the lowermost portion of the ferromagnetic tubular member. The ferromagnetic tubular member is advanced through the bin of dry magnetic particles in a helical path to ensure that the entire outer periphery of the tubular member passes close to the bin of dry magnetic particles. The magnetic particles are attracted to the tubular member and are subject to detection as the part advances helically through stray magnetic fields which are generated by defects in the outer periphery of the tubular member.
There are several serious disadvantages with this approach, including: